我創(chuàng)建了 3 個(gè)下拉列表，我想按學(xué)生姓名、父親姓名或序列號(hào)進(jìn)行搜索。當(dāng)我選擇學(xué)生姓名或父親姓名并寫下姓名時(shí)，它會(huì)顯示數(shù)據(jù)。但是當(dāng)我選擇“按序列號(hào)”搜索時(shí)，出現(xiàn)錯(cuò)誤：已經(jīng)有一個(gè)DataReader與此命令關(guān)聯(lián)的打開，必須先關(guān)閉。錯(cuò)誤代碼搜索截圖我試圖關(guān)閉 dataReader 但它不起作用。protected void btnSearch_Click(object sender, EventArgs e){        string QueryStudent = "Select SerialNumber as [Serial No.], Name as [Student Name], FatherName as Father,Phone,Course, AdmissionDate as[Admission date], Status from tblStudents where Name= '" + txtSearch.Text + "'";        string QueryFather = "Select SerialNumber as [Serial No.], Name as [Student Name], FatherName as Father,Phone,Course, AdmissionDate as[Admission date], Status from tblStudents where FatherName= '" + txtSearch.Text + "'";        string QuerySerial = "Select SerialNumber as [Serial No.], Name as [Student Name], FatherName as Father,Phone,Course, AdmissionDate as[Admission date], Status from tblStudents where SerialNumber= '" + txtSearch.Text + "'";        con.Open();        using (var sqlCommand = new SqlCommand(QueryStudent, con))        using (var sqlCommand2 = new SqlCommand(QueryFather, con))        using (var sqlCommand3 = new SqlCommand(QuerySerial, con))        {            SqlDataReader reader = sqlCommand.ExecuteReader();            if (DropDownList.SelectedItem.Value == "By Student Name")            {                if (!reader.HasRows)                {                    GrindSearch.Visible = false;                    lblmsg.Visible = true;                    lblmsg.Text = "No Record found";                }